DESCRIPTION Tuberculosis remains an international health problem of immense proportions, as one-third of the world's population is currently infected with the causative organism, Mycobacterium tuberculosis. Cell-mediated immunity, involving the interaction of sensitized lymphocytes with M. tuberculosis-infected mononuclear phagocytes, can contain the organism, resulting in the finding that the great majority of otherwise healthy individuals do not develop active tuberculosis following infection. HIV-positive individuals have greatly increased susceptibility to the development of active disease following infection with M. tuberculosis. Furthermore, tuberculosis is often an early complication of HIV infection which precedes marked declines in CD4+ T-cell counts. In contrast to studies of other intracellular pathogens, in vitro addition of activating cytokines does not mediate significant reduction in the intracellular growth of M. tuberculosis. Utilizing a method of quantitative infection of human mononuclear phagocytes with virulent M. tuberculosis in which the interactions of lymphocytes with these infected cells can be characterized, preliminary studies indicate that addition of lymphocytes to cultures of M. tuberculosis-infected monocytes is more effective at limiting intracellular growth than is transfer of the supernatants of those cultures. It is hypothesized that cell-to-cell contact involving the interactions of specific cell surface molecules is necessary to maximally activate human mononuclear phagocytes to contain intracellular growth of M. tuberculosis. This hypothesis will be addressed in the following Specific Aims: 1) To determine the contact-dependent mechanisms by which antigen-specific CD4+ T-cells activate bactericidal functions of M. tuberculosis-infected mononuclear phagocytes; 2) To determine whether maximal CD4+ T cell-mediated activation of M. tuberculosis-infected mononuclear phagocytes requires sequential or interactive effects of cell contact and cytokine production; and 3) To determine whether CD8+ T-cells, gammadelta T-cells, and NK cells mediate contact-dependent activation of M. tuberculosis-infected mononuclear phagocytes, to compare the mechanisms used by these effector populations to those of CD4+ T cells, and to determine the role of Fas/FasL-mediated cytotoxic effector mechanisms on killing of M. tuberculosis within human mononuclear phagocytes.